Electroluminescent devices (hereinafter also referred to as EL devices) contain spaced electrodes separated by an electroluminescent medium that emits electromagnetic radiation, typically light, in response to the application of an electrical potential difference across the electrodes. The electroluminescent medium must not only be capable of luminescing, but must also be capable of fabrication in a continuous form (i.e., must be pin hole free) and must be sufficiently stable to facilitate fabrication and to support device operation.
Initially organic EL devices were fabricated using single crystals of organic materials, as illustrated by Mehl et al U.S. Pat. No. 3,530,325 and Williams U.S. Pat. No. 3,621,321. Because single crystal organic electroluminescent layers were relatively difficult to fabricate and further did not readily lend themselves to thin layer constructions in thicknesses below about 50 .mu.m, the art turned to the use of thin film deposition techniques to form the organic layer of EL devices. Unfortunately, thin film deposition techniques produced devices which exhibited performance efficiencies 1 to 2 orders of magnitude below that obtained with single organic crystal devices.
In the last decade the art has developed a new class of organic EL devices hereinafter referred to as internal junction organic EL devices which lend themselves to thin film deposition techniques for fabrication of the organic layers and which exhibit performance characteristics comparable to or better than those of single organic crystal EL devices. This new class of organic EL devices has been made possible by dividing the organic medium separating the electrodes into a hole injecting and transporting zone and an electron injecting and transporting zone. The interface of the two organic zones constitute an internal Junction allowing injection of holes into the electron injecting and transporting zone for recombination and luminescence, but blocking electron injection into the hole injecting and transporting zone. Examples of internal Junction organic EL devices are provided by Tang U.S. Pat. No. 4,356,429, VanSlyke et al U.S. Pat. Nos. 4,539,507 and 4,720,432, and Tang et al U.S. Pat. Nos. 4,769,292 and 4,885,211.
One area of concern in the performance of internal Junction organic EL devices has been the decline of luminescence during the operating life of the device. If the device is driven at progressively higher voltages to keep luminescence to an invariant level, eventually a voltage level is required that cannot be conveniently supplied by the driving circuitry or which produces a field gradient (volts/cm) exceeding the dielectric breakdown strength of the layers separating the electrodes, resulting in a catastrophic failure of the device.
VanSlyke et al U.S. Pat. No. 4,539,507 recognized that tertiary amines (including diamines) containing phenyl or phenylene groups when employed to form the hole injecting and transporting zone of an internal Junction organic EL device increased the stability of light output and thereby increased operating life. VanSlyke et al U.S. Pat. No. 4,720,432 recognized that still higher levels of stability could be realized by fabricating the hole injecting and transporting zone as two layers: a hole injecting layer contacting the cathode and a contiguous hole transporting layer forming a Junction with the electron injecting and transporting zone. VanSlyke et al '432 employed a tertiary amine in the hole transporting layer and a porphyrinic compound of the type disclosed by Tang U.S. Pat. No. 4.356,429 in the hole injecting layer.
Brantly et al U.S. Pat. Nos. 3,567,450 and 3,658,520 (cited by VanSlyke et al U.S. Pat. No. 4,720,432) disclose triarylamines useful in electrophotographyic systems. At least one of the aryl substituents is further substituted and can be either a phenylene or naphthalene group.
Despite the improvements in internal Junction organic EL device operating life and overall luminescence afforded by the VanSlyke et al discoveries, the luminescence of internal junction organic EL devices still declines initially at a comparatively high rate. When a device is driven at a constant current, luminescence often declines sharply during the first few hours of operation with luminescence declining at a slower rate thereafter. For example, a typical EL device will exhibit half of its total luminescence loss over 300 hours of operation within the first 10 to 20 hours of operation, and most of the loss of luminescence within the first 10 to 20 hours occurs in the first 1 to 2 hours of operation.